The present invention relates generally to the assignment of wireless communications mobile terminals to particular cells, and more particularly to the assignment of mobile terminals to particular cells during group calls.
Wireless communications mobile terminals, such as cellular phones and the like, provide a wide variety of services. One increasingly popular service is so-called conference or group call service, wherein a plurality of users having different terminals (mobile or otherwise) are connected together and able to transmit and receive to all of the other active members of a user group. To implement group call functionality in wireless communications system, a user group identification (UGID) code is typically assigned to each of a number of different subsets of mobile terminals. See, for example application Ser. No. 09/192,185, entitled xe2x80x9cUser Group Indication and Status Change In Radiocommunications Systems,xe2x80x9d which is incorporated herein by reference.
From a system point of view, one advantage of group calls is that multiple mobile terminals may be able to share a single downlink traffic channel within a given cell, rather than having to have separate downlink traffic channels assigned for each mobile terminal. Further, multiple mobile terminals within a given cell involved in a group call may also be able to share a single uplink traffic channel with some technologies. Focusing on just the downlink channel for simplicity of illustration, assume five mobile terminals (MT1-MT5) are active in the group call and present in the same cell (C1) and two other mobile terminals (MT6 -MT7) are active in the same group call in another cell (C2). All the mobile terminals in one cell (e.g., C1 or C2) are typically able to share a single downlink traffic channel assigned to the group call session, rather than having to have separate downlink traffic channels assigned to each mobile terminal. This may result in only two traffic channels being usedxe2x80x94one in each cellxe2x80x94rather than seven different traffic channels. From a system point of view, the five channels not used may be assigned to other tasks, thereby allowing for greater overall system capacity utilization.
From time to time during a group call session, a mobile terminal may move from one cell""s geographic area to another cell""s geographic area and/or the signal quality in a given cell may degrade to a point approaching unacceptable levels. In either case, it may be necessary for the mobile terminal to be assigned to a different cell in order to maintain acceptable performance. That is, the mobile terminal may need to change from communicating with the wireless communications system via base station X to communicating with the wireless communications system via base station Y (or the equivalent in satellite based systems). In circuit switch systems, this process is typically referred to as xe2x80x9chandoff,xe2x80x9d and is typically controlled by the wireless communications system in a manner well known in the art. In packet data systems, this process is typically referred to as xe2x80x9ccell reselection,xe2x80x9d and is typically controlled by the mobile terminal in a manner likewise well known in the art.
If the result of the change in cell assignment is that the mobile terminal is assigned to a cell not otherwise actively engaged in the group call, the new cell must allocate a traffic channel under its control to the group call so that the xe2x80x9cnewly arrivedxe2x80x9d mobile terminal may continue to participate in the group call. Thus, in the example given above, if mobile terminal MT3 is moving out of cell C1, it will eventually need to have its cell assignment changed via what is known generically as a cell assignment change procedure. If the result is MT3 being assigned to a new cell C3, then cell C3 must allocate a traffic channel to MT3 so that MT3 can remain active in the group call. In such a situation, the total number of traffic channels needed for the group call session would increase from two to three. Because more channels are being consumed by the group call, the overall capacity of the wireless communications system is reduced. If, on the other hand, mobile terminal MT3 is instead assigned to cell C2, where MT6 and MT7 are already present and active in the group call, then MT3 may simply join MT6 and MT7 on their shared channel. In this scenario, the number of channels used by the group call does not change, and the change in cell assignment for MT3 results in essentially no net loss in capacity to the wireless communications system.
From the above, it is clear that minimizing the number of cells that mobile terminals involved in a given group call are assigned to helps preserve the overall capacity of a wireless communications system. However, the existing cell assignment procedures do not take group call participation into account when assigning a mobile terminal to a cell. Thus, there is a need for an improved cell assignment procedure that recognizes that mobile terminals that are active in a group call should be kept in the same cells to the extent reasonably possible.
The cell assignment process of the present invention strives to minimize the number of cells assigned to mobile terminals involved in any given group call session. When a cell assignment trigger event, also known as a handoff trigger event or a cell reselection trigger event, is encountered during a group call, the decision whether to change cells or not is based at least in part upon whether the relevant cell is already participating in that group call session with another mobile terminal. If so, the cell assignment process preferentially selects such a cell for switching to, or avoids changing cell assignment if already assigned there.
In one embodiment, the present invention preferentially selects cells already participating in the group call session when changing cell assignments. In other embodiments, the present invention avoids reassigning the mobile terminal in response to encountering a trigger event when several conditions are met. These conditions include, generally, the current cell both having at least minimally acceptable performance and currently supporting at least one other mobile terminal of the same group call session, and also that no candidate cells are currently supporting the same group call session. If the three conditions are met, the cell assignment of the mobile terminal is not changed; otherwise the cell assignment of the mobile terminal is changed. In addition, other embodiments of the present invention actively seek out other cells involved in the group call session and seek to change the cell assignments of the mobile terminals involved in the group call session to those cells, even when the signal quality may be acceptable in the current cell. Thus, the mobile terminals involved in the group call session are gradually urged toward congregating in the fewest possible cells under the circumstances.
The decision whether to make the new cell assignment may be made in the mobile terminal or in the wireless communications system with the present invention. Further, the present approach functions in both circuit-switched and packet-data environments. Additionally, one optional aspect of the present invention is to give mobile terminals involved in group call sessions preferential priority when switching cells so as to help minimize disruption to the group call session. Thus, the present invention helps provide better service with less of a system capacity penalty for group call sessions.